This invention relates to rotary actuators, and more particularly to limited angle actuators such as limited angle torque (LAT) motors and rotary solenoids.
Rotary actuators are known which provide torque over a predetermined angle. Typical prior art actuators have a rotor including permanent magnets and a stator that supports armature windings. Although conventional brushless motors are constructed in a similar manner to limited angle torque (LAT) actuators and motors, LAT actuators are typically wound in single phase to eliminate the need for commutation circuitry. Conventional brushless motors are typically wound for two or three-phase operation.
Two basic designs of LAT motors are known in the prior art. In the slotted-armature LAT motor, the armature windings are embedded in slots around the inside periphery of a laminated stator.
In the torroidally-wound LAT motor, the coils are torroidally wound on a slotless stator. The rotor carries one or more permanent magnets which interact with the magnetic field created by the current-carrying coils to produce torque over a limited excursion angle. In a typical prior art device, the excursion angle is about 45 degrees or less.
One disadvantage of such prior art LAT actuators is that the torque they produce is constant only over a very limited range. The torque is inversely proportional to the distance or "air gap" between the stator's magnetic poles and the magnetic poles of the permanent magnet carried on the rotor. As the rotor rotates to move a permanent magnet pole away from a stator pole, a smaller portion of the stator's flux is transmitted to the permanent magnet's pole, thereby decreasing the torque output. This decrease in torque output also limits the actuator's excursion angle.
One attempt to remedy this problem is disclosed in U.S. Pat. No. 3,694,782 issued Sept. 26, 1972 to Ray. In the '782 Ray patent, the permanent magnet on the rotor has peripheral segments of highly permeable material attached thereto. These segments or shoes may have a tapered or wrapped design to increase the amount of permeable material that receives magnetic flux from the stator as the permanent magnet pole moves away from the section of the stator having the highest flux density.
However, the actuator in the Ray patent still has several disadvantages. First, the Ray excursion angle is limited to between about 40 degrees to 55 degrees. Second, the Ray design still does not provide constant torque over a wide range.